selftests/bpf: Add {load,search}_kallsyms_custom_local()

These two functions allow selftests to do loading/searching
kallsyms based on their specific compare functions.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240326041513.1199440-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>